The invention relates to an improved composition for use as a liner layer in a rocket motor. Rocket motors employing solid propellants typically comprise a rigid outer casing or shell; a heat insulating layer (insulation) bonded to all or part of the inner surface of the casing; a liner layer (liner) bonded to the insulating layer; and a solid propellant grain bonded to the liner. The insulation is generally fabricated from a composition capable of withstanding the high temperature gases produced when the propellant grain burns, thus protecting the casing (or selected parts of it). The liner is an elastomeric composition which bonds the solid propellant grain to the insulation and to any uninsulated portions of the casing.
Improvements in propellant processing taught in U.S. Pat. Nos. 4,110,135 and 4,184,031, respectively issued to Graham, et al. on Aug. 29, 1978 and Jan. 15, 1980, which patents are incorporated herein by reference, reduce the propellant cure time and increase its pot life. Thus, the propellant can be mixed, stored, and worked for relatively long periods of time without prematurely curing, but once the cure is initiated it proceeds rapidly to completion. Graham, et al. achieved these advantages by using a propellant binder system comprising hydroxyl terminated polybutadiene based polyurethane (the reaction product of a hydroxyl terminated polybutadiene prepolymer and a polyisocyanate curing agent) and a curing catalyst comprising a mixture of magnesium oxide, maleic anhydride or maleic acid, and triphenyl bismuth.
For Graham, et al., a carboxylic acid anhydride such as maelic anhydride is suitable only because it is convertible to a carboxylic acid. This is because the catalyst is a metal salt produced by the reaction of triphenyl bismuth with a carboxylic acid and magnesium oxide is utilized to regulate the rate of catalysis.
Thus, when a carboxylic acid anhydride is used, that anhydride must first be converted to a carboxylic acid. The thus formed carboxylic acid must be sufficiently long lived to react with the triphenyl bismuth to form the operative metal salt catalyst.
Based on Graham, et al., a carboxylic acid anhydride, triphenyl bismuth, magnesium oxide curing catalyst would not be expected to work in an alkaline environment. First, because alkalinity would not be conducive to the formation of a carboxylic acid. Secondly, because alkalinity would be expected to neutralize any carboxylic acid present and prevent the formation of the Graham, et al. metal salt catalyst. Graham, et al. also teaches the use of carbon black as an optional propellant additive. However, there is no disclosure or teaching or suggestion to use alkaline carbon black which provides an alkaline propellant environment.